An MRI apparatus is an apparatus that applies a gradient magnetic field and an excitation radio frequency magnetic field to a subject disposed in a uniform static magnetic field space in a shield room, receives a nuclear magnetic resonance signal generated using a nuclear magnetic resonance phenomenon by a radio frequency coil (RF reception coil), and images a test object. A range capable of being simultaneously imaged is limited to the range of the static magnetic field space at most, and a range where high image quality is obtained is limited to the sensitivity range of the RF reception coil.
As performance characteristics which are necessary in the RF reception coil, there are SN ratio for high image quality, wide sensitivity range for wide-field imaging, parallel imaging performance for high speed imaging, and the like. The parallel imaging is a method for simultaneously performing signal measurement using a reception coil formed by plural coil elements to reduce an imaging time. Plural rectangular or circular loop-shaped coil elements are arranged in a certain direction (a body width direction or a body length direction), and a phase encoding direction is set along the direction. In this technique, if the arrangement of the coil elements is optimal, it is possible to reduce the imaging time to the time divided by the number of the coil elements arranged in the phase encoding direction. Further, by two-dimensionally arranging the plural loop-shaped coil elements (for example, in the body width direction or the body length direction), it is possible to realize a reception coil capable of achieving higher-speed imaging and having a sensitivity region in a wide range such as the whole body.
In order to realize excellent parallel imaging, it is necessary that electromagnetic coupling between the plural loop-shaped coil elements is sufficiently small. This is because if the electromagnetic coupling between the coil elements is present, noise interference occurs between the coil elements, and thus, the SN ratio of an image deteriorates. In order to solve such a problem, in a method disclosed in NPL 1, magnetic coupling that occurs between elements is suppressed by using an amplifier with low input impedance and a capacitor connected to each element for signal detection and amplification. According to this method, if the distance between two coil elements is long to a certain degree, it is possible to reduce the electromagnetic coupling between two coil elements to a degree without a practical problem. Here, when the loop size of the coil element is large with respect to the distance between two coil elements, the magnetic coupling cannot be suppressed by only the method disclosed in NPL 1. In this case, by appropriately overlapping two adjacent coil elements (about 10% in area), it is possible to remove the magnetic coupling between the coil elements. When the degree of overlapping is not appropriate, a resonance point of input impedance of the coils is divided into two or more because of inductive coupling between the coil elements. When the electromagnetic coupling between two coil elements is large as the resonance point of input impedance of the coils is divided into two or more, the magnetic coupling cannot be suppressed, even using the method disclosed in NPL 1. Accordingly, in reality, it is preferable that coupling is reduced to a sufficiently small degree by using the method disclosed in NPL 1 and the overlapping method together. Further, by connecting in series an auxiliary coil to each of two coils for which coupling is to be reduced, it is possible to remove inductive coupling between coils.
The above description is mainly made with respect to the RF reception coil that receives a nuclear magnetic resonance signal, but the same coil structure may be realized with respect to an RF transmitting coil for application of a radio frequency magnetic field. In the case of the transmitting coil, a power amplifier with low output impedance and a pulse generator (transmission modulator), instead of a low noise amplifier with low input impedance for signal detection and amplification and a receiver, are connected to the RF coil. As performance characteristics which are necessary in the transmitting coil, there are high emission efficiency for a low specific absorption rate (SAR), uniform magnetic field generation performance, and the like. For this purpose, by supplying radio frequency magnetic fields having different amplitudes and phases to plural coil elements, or by controlling an element that supplies a radio frequency magnetic field so that only a desired portion is irradiated with an excitation magnetic field, it is possible to make an excitation magnetic field distribution for a subject uniform, or to reduce the specific absorption rate (SAR). Such a technique is referred to as RF shimming or parallel transmission. In this case, similar to the case of the reception coil, it is necessary that electromagnetic coupling between coil elements is sufficiently small. In consideration of different forms of use of a coil that includes plural elements, if a transmitter-receiver switching circuit is used between a reception amplifier, a transmission amplifier, and the coil, it is possible to use the coil as a RF coil used both as transmission coil and reception coil. Accordingly, since the following description relates to a coil capable of being used as a transmitting coil, and also, as a reception coil, an “RF coil” is used as a term including two meanings of the “RF reception coil” and the “RF transmitting coil”.
However, when testing a wide range all at once, a wide range RF coil in which the plural loop-shaped coil elements as described above are two-dimensionally arranged (for example, in the body width direction and the body length direction) to widen a sensitivity range is used. Further, when testing a local range such as a head or a shoulder with high definition, an RF coil dedicated to each portion (dedicated to the head in the case of the head, or dedicated to a shoulder joint in the case of the shoulder) is used. Even in the case of the RF coil dedicated to each portion, the RF coil includes plural coil elements, and the coil elements are provided in a unit having a shape dedicated to a portion of the subject to be imaged, and are optimally arranged therein so that electromagnetic coupling is suppressed to the minimum.
The wide range RF coil and the RF coil dedicated to each portion as described above are appropriately set on a top plate by an operator at every instance of imaging, in the related art. It is necessary that the operator carries the RF coil and correctly aligns a subject and the RF coil at every instance of imaging. Particularly, in the case of the RF coil having the above-described wide sensitivity range, since the size is large and the weight is heavy, the workload of the operator who carries the RF coil is increasing. Further, when a positional relationship between the subject and the coil is not correct, it is necessary to separate the subject from the coil and to perform resetting.
From such a background, as requirements for the RF coil, a requirement of enhanced usability is increased in addition to requirements such as high image quality, high speed imaging, a low SAR, or uniform emission magnetic field control, and there is demand for an RF coil in which coil setting at every instance of imaging is unnecessary. Further, as disclosed in PTL 1, an RF coil that is permanently mounted on a cradle (a stand or a top plate) that covers a subject or is built in the cradle has been proposed. Further, a whole body photographing method that employs parallel imaging and table movement together has been used. Since the RF coil used in such a case is an RF coil which is formed by arranging plural stereoscopic elements such as saddle coils disclosed in PTL 1 or two-dimensionally arranging the above-mentioned loop shaped coil elements, and has a wide sensitivity range. The wide range RF coil that includes the plural elements is permanently mounted on a table, and a part of the elements is appropriately selected and controlled so as to be operated after moving to the position of the cradle and the position of the center of a magnetic field. Thus, it is possible to image a wide range such as the whole body in a seamless manner. Further, in the case of the RF coil dedicated to each portion, if the RF coil is permanently mounted at a determined position on the cradle together with the wide range RF coil, similarly, by appropriately selecting and controlling an element present at the position of the top plate and the position of the center of the magnetic field, it is possible to perform imaging in a seamless manner. In both cases, a “lower coil” disposed under a subject in a body thickness direction is built in the cradle, or is permanently mounted in the cradle, and thus, a patient setting time is reduced.
Further, when setting a subject in the RF coil that is permanently mounted in the cradle, in the case of the “lower coil” disposed under the subject in the body thickness direction, it is possible to set the subject with the “lower coil” being permanently mounted. However, in the case of an “upper coil” disposed above the subject in the body thickness direction, the “upper coil” becomes an obstruction in setting the subject. Thus, when setting of the subject, by employing a structure in which the “upper coil” and the “lower coil” are separated or the “upper coil” slides with respect to the “lower coil”, it is possible to easily perform the setting of the patient and the coil.
The above-described RF coil is configured so that its shape is not changed, but in order to realize a higher SN ratio or emission efficiency, a flexible structure such that the coil is in contact with a subject may be used. In the related art, in order to enhance convenience when setting a patient, even in the case of a lower coil of a permanently mounted wide range RF coil being used in a flat shape, if a part thereof is formed as a flexible structure, it is advantageous to wind the coil around a subject according to each portion to be imaged. Further, if the RF coil dedicated to each portion is formed as a flexible structure, it is possible to perform imaging with respect to a movement of a joint or the like as an imaging object, and even when the size of the imaging object becomes different, it is possible to perform imaging in a coil shape of being constantly in contact therewith. In performing imaging using the RF coil having such a flexible structure, in order to obtain an image of higher image quality, a patient fixture belt for fixing a positional relationship between the coil and a subject is used. PTL 2 discloses a fixing belt for fixing a subject on a table (bed), in which an RF coil is disposed to be at least partially assembled with the fixing belt. Further, the RF coil assembled with the fixing belt is coupled with an electronic device in the table by conductive coupling, or is coupled with the electronic device in the table by capacitive coupling or inductive coupling, and thus, convenient patient setting is realized. Further, PTL 3 discloses a structure in which a lower coil that is permanently mounted on a top plate on a table is at least partially deformable with respect to a subject placed on the lower coil so as to select an imaging position and a non-imaging position. Thus, in imaging, the subject and the coil are in contact with each other to realize a high SN ratio, and in non-imaging, the lower coil becomes flat so that setting of the subject is easily performed.
Generally, a transmitting coil generates a reflected wave if matching between the transmitting coil and a load is not sufficient, and thus, its emission efficiency is lowered. PTL 4 discloses a technique for solving the problem. In this technique, plural capacitor banks are provided to be respectively switched, and even when a load varies, reflection is suppressed to prevent reduction in emission efficiency.